1. Field of the Invention
This invention relates to the curing of liquid polysulphide polymers and in particular to the use of isocyanate curing agents to prepare thin films, coatings, adhesives and mouldings of polysulphide polymers and compounds.
2. Description of the Prior Art
Compositions based on liquid polysulphides have a wide range of applications as sealants, strips, sheets, films, spray-on coatings and the like. Many such formulations include carbon black as a filler and reinforcing agent.
Liquid polysulphide resins are usually cured with an oxidising agent such as manganese dioxide, the cure mechanism being as follows: ##STR1##
Other mechanisms for polysulphides have been tried. For example, polymers have been made by reacting polysulphides with isocyanates using an amine catalyst. However, water isocyanate reactions were prevalent in these systems, the carbon dioxide produced resulted in a blown product. ##STR2## Subsequently, A Can react with
i) further isocyanate PA1 ii) the reactive end of active polymer (B) PA1 i) further polysulphide mercaptans PA1 ii) the unreactive end of reactive polymer (A) PA1 iii) moisture in the system PA1 i) high shear strength PA1 ii) high resilience to withstand impact PA1 iii) ease of application PA1 iv) transparency or translucence PA1 v) non stress-crazing of polycarbonate PA1 vi) durability to the environment
B Can react with
In amine catalysed systems the isocyanate reacts with water in the atmosphere in preference to the mercaptan groups. ##STR3## The amino group formed autocatalyses the reaction and an exotherm occurs. With this catalysis the reaction between isocyanate and mercaptan is preferred.
Nevertheless, problems still arise in the preparation of thin films and coatings. In particular the curing reaction can proceed too rapidly and exothermically and is not easily controlled. There is still a potential problem of blowing resulting from reactions between isocyanate groups and water in the system or in the air.